My advice: know the people you're travelling with!!
We always stay at least 10 nights. Our reasons are:
1. DH has a stressful job. It takes him a couple of days at the beginning of a trip just to unwind. Anything less than 8 days and it's hardly worth him going.
2. DS (16). Typical teenager. Wants to close the parks down and sleep late the next morning. Would be content to stay in the hotel's arcade! Also loves the water parks.
3. DS (12). Total water puppy.

Give him a pool and he's happy. SAB was total nirvana for him! He had to have some time at the pool every day. Would rather go to the water parks than the theme parks.
4. Me...I'm happy. I'm at WDW, right? I won't take the boys out of school, for that long...so we go during the summer!

Factor in the crowds and heat, and we need at least 10 nights to accomodate everyone and feel like we had any kind of vacation at all!
Since we feel like we have to spend that kind of time, we only go once every two years. It's more important for us to space our visits out a little bit and be able to enjoy ourselves when we do get to go!!!!